Recently, a secondary battery such as a lithium ion battery has been used in various fields so as to reduce environmental load. Deterioration of the secondary battery progresses while in use. To reduce the running cost of the secondary battery, it is necessary to suppress the progress of the deterioration.
Patent Document 1 and Patent Document 2 disclose a method of stably supplying electric energy for a long period of time by alternately charging and discharging two or more sets of secondary batteries so as to increase operational lifetime of the secondary batteries.